Gearing-torque division

ABSTRACT

A transmission comprising a drive shaft for driving a main section of the transmission, a driven shaft driven by the main section so as to drive in turn a range section, and an output shaft driven by the range section. Each section has a pair of intermediate shafts, the plane containing the axes of one pair being angularly offset from the plane containing the axes of the other. Each intermediate shaft carries a helical gear meshed with a common helical gear on the section input shaft. The main section includes a series of gear assemblies providing different gear ratios, each assembly comprising a helical gear on the driven shaft meshed with a respective pair of helical gears carried by respective intermediate shafts. In the range section each intermediate shaft carries a double helical gear, each double helical gear being meshed with single helical gears on the output shaft. The helical gears on the output shaft and the drive shaft are each mounted so as to permit pivoting about an axis normal to the shaft axis to enable balancing of axial components of reaction forces, and thus transmission of torque via all intermediate shafts simultaneously. Each of the plurality of gear assemblies in the main section is individually adjusted so that simultaneous contact of the driven shaft gear with its intermediate shaft gears is achieved when the main section pivotal gear is in a predetermined disposition. The angles of inclination of the teeth of the gear assemblies are different to facilitate transmission of torque via both intermediate shaft if the transmission should go into overrun. Each intermediate shaft of the main section extends through to the range section, and carries a helical idler gear meshed with the helical gear at the input end of the adjacent range section intermediate shaft. The idler gears can be clutched to their respective main section intermediate shaft to permit reverse rotation of the output shaft for unidirectional rotation of the drive shaft.

United States Patent [1 1 Pengilly [451 May 27, 1975 GEARING-TORQUEDIVISION [76] lnventor: Eric Alexander Pengilly,

[22] Filed: Apr. 27, 1973 [21] Appl. No.: 354,955

[30] Foreign Application Priority Data 7 Apr. 29, 1972 United Kingdom20002/72 [52] US. Cl. 74/331; 74/359; 74/333; 74/745; 74/410; 74/361;74/355 [51] Int. Cl. F16h 3/08; Fl6h 57/02 [58] Field of Search 74/359,360, 331, 363, 74/333, 745

[56] References Cited UNITED STATES PATENTS 3,046,807 7/1962 Barth etal. 74/359 X 3,105,395 10/1963 Perkins 74/331 3,237,472 3/1966 Perkinset a1. 74/331 3,335,616 8/1967 Perkins 74/331 3,425,290 2/1969 Perkins74/331 3,500,695 3/1970 Keiser 74/359 X 3,611,823 10/1971 Richards74/331 Primary ExaminerSamuel Scott Assistant Examiner-P. S. LallAttorney, Agent, or FirmOblon, Fisher, Spivak, McClelland & Maier [57]ABSTRACT A transmission comprising a drive shaft for driving a mainsection of the transmission, a driven shaft driven by the main sectionso as to drive in turn a range section, and an output shaft driven bythe range section. Each section has a pair of intermediate shafts, theplane containing the axes of one pair being angularly offset from theplane containing the axes of the other. Each intermediate shaft carriesa helical gear meshed with a common helical gear on the section inputshaft. The main section includes a series of gear assemblies providingdifferent gear ratios, each assembly comprising a helical gear on thedriven shaft meshed with a respective pair of helical gears carried byrespective intermediate shafts. In the range section each intermediateshaft carries a double helical gear, each double helical gear beingmeshed with single helical gears on the output shaft. The helical gearson the output shaft and the drive shaft are each mounted so as to permitpivoting about anaxis normal to the shaft axis to enable balancing ofaxial components of reaction forces, and thus transmission of torque viaall intermediate shafts simultaneously. Each of the plurality of gearassemblies in the main section is individually adjusted so thatsimultaneous contact of the driven shaft gear with its intermediateshaft gears is achieved when the main section pivotal gear is in apredetermined disposition. The angles of inclination of the teeth of thegear assemblies are different to facilitate transmission of torque viaboth intermediate shaft if the transmission should go into overrun. Eachintermediate shaft of the main section extends through to the rangesection, and carries a helical idler gear meshed with the helical gearat the input end of the adjacent range section intermediate shaft. Theidler gears can be clutched to their respective main sectionintermediate shaft to permit reverse rotation of the output shaft forunidirectional rotation of the drive shaft.

19 Claims, 8 Drawing Figures SHEET FIGS ZOO

PATENTEDMAYZ? 1925 3, 885MB SHEET 2 GEARING-TORQUE DIVISION INTRODUCTIONThe present invention relates to transmissions.

The design of gearboxes for heavy goods vehicles is continually subjectto demand for transmission of more power. The conventional gearboxtransmits power between main shafts by way of a single intermediateshaft. In order to transmit more power with this design, it is necessaryto increase the center distances between the main shafts and theintermediate shaft, thus necessitating the use of a larger casing whichtakes up more space in the overall vehicle design, and larger gears.Increased center distance also involves increased load on the bearings,so that bearing design may become the limiting problem in the gearboxdesign. Further, with larger gears, the pitch line speed will increasebecause the input shaft will rotate at a substantially constant speed.Increased pitch line speed carries with it increased noise in operation,and this noise is at present becoming a serious problem in gearboxdesign.

PRIOR ART A solution is the use of a plurality of intermediate shaftsspaced around the main shaft. If the load torque is equally sharedbetween these intermediate shafts there is no necessity to increase thegear sizes, or gear sizes could be reduced. However, this ideal solutionis not readily obtainablein practice, because it is impossible tomanufacture the parts of the transmission, and to assemble them withsufficient accuracy to ensure that full load torque is satisfactorilyshared between the two intermediate shafts. In practice, it is foundthat a gear on one or other of the shafts will engage first with thecorresponding main shaft gear, and substantially the full load will betransmitted via the engaged gear and its corresponding shaft.

This problem arises because of a multitude of factors which affect therelationship of the parts within the assembly, for example base pitcherrors, the location of the teeth of each gear relative to the key wayon the associated shaft, the mounting of each shaft in its bearings, andmany other factors. As indicated above, it is not a practicalproposition to control all these factors in the assembly stage. 1

Several different attempts at solutions to this problem have been made.One such attempt (see for example US. Pat. Nos. 3,425,290; 3,335,616;3,105,395 and 3,237,472) has been in connection with gear trainsemploying spur gears. This arrangement involves the provision of adegree of radial freedom for a main shaft or a main shaft gear. Thus, ifonly one intermediate shaft is transmitting torque, the radial load onthe free shaft or gear will move that element towards the otherintermediate shaft to an extent sufficient to force that shaft totransmit torque so as to balance the radial loads on the free element.However, this freedom contributes to, rather than mitigates, the noiseproblem.

Alternative proposals for gear trains transmitting very high power forexample in marine engines, have employed helical gears developingaxially directed reaction components in the intermediate shafts. Theintermediate shafts have been extended through the gearbox casing toco-operate with a balancing coupling outside the gearbox. The coupling,which may comprise a pivoted lever or a complex hydraulic circuit, isarranged to cause axial movement of an intermediate shaft which is nottransmitting load so as to force a helical gear carried by that shaftinto Ioadtransmitting contact with a main-shaft gear, thereby to balanceaxially directed reaction forces in the intermediate shafts. However,these proposals each involve additional equipment outside the gearboxcasing adding to space requirements and expense of the gearbox as awhole, and they also involve freedom of each intermediate shaft in itsbearings.

SUMMARY OF THE INVENTION According to the present invention, there isprovided a transmission comprising first and second rotatable members, afirst helical gear rotatable with said first member, a plurality offurther helical gears meshed with said first gear and coupled with saidsecond member, and mounting means for said first gear arranged to permitit to pivot under the action of axially directed components of reactionforces arising from said further gears under load so that torque can betransmitted between the members in each of said further helical gears.

The coupling means may comprise a plurality of gear assemblies providingdifferent gear ratios, each assembly comprising a plurality of gearsfixed for rotation with respective ones of said further gears and anassociated gear, there being clutching means for selectively clutchingthe associated gears with the second member. However, a problem mayarise if an unclutched assembly binds before each of the plurality ofgears in a clutched assembly has achieved contact with the associatedgear. The transmission can be assembled so as to avoid this problem iftorque is transmitted always from one particular member to the other,but additional means may be required if either member can act as thedriving member.

According to a second aspect of the invention, therefore, there isprovided a transmission comprising first and second members, a pluralityof gear assemblies providing different ratios, each assembly comprisinga plurality of gears each coupled with one of said members and anassociated gear meshed with each of the plurality of gears in itsassembly, and clutch means for selectively clutching said associatedgears to the other of said members, each of said gear assembliescomprising helical gears and the helix angles of the teeth of said gearassemblies being different so that for any of said assemblies, when itsassociated gear is clutched to said other member, torque can betransmitted via each of the plurality of gears both from said firstmember to said second member and from said second member to said firstmember, the larger the meshing radius of the gears of the plurality, thelarger the helix angle of the teeth of these gears.

A third aspect of the invention is directed to an arrangement of atransmission to provide a reversible drive from a source ofunidirectional, rotational drive, particularly when the transmission hasa main section and a range section for obtaining an increased number ofpossible speed changes.

A conventional transmission for providing reversible drive employs areverse gear assembly additional to those provided to obtain forwardspeeds. This reverse assembly includes a reverse idler mounted on anadditional shaft (see for example US. Pat. No. 3,335,616 referred toabove).

According to the third aspect of the present invention, there isprovided a transmission for obtaining reversible drive from a source ofunidirectional, rotational drive comprising an input member forconnection with said source, an output member to be reversibly driven,an output member driving gear for driving said output member, first andsecond couplings between said gear and said input member arranged todrive said gear in opposite directions and clutch means for selectingwhich of said couplings is effective to drive the gear.

The first coupling may comprise a first gear rotatable by said inputmember, a second gear meshed with the first and a third gear meshed withsaid driving gear, the second coupling comprising said first gear and afourth gear meshed with the driving gear, said clutch means beingarranged to select which of said third and fourth gears is rotatable bysaid first gear to drive said driving gear.

BRIEF DESCRIPTION OF THE DRAWINGS By way of example, some embodiments ofthe invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a diagramatic, sectioned plan view of a simple transmissionembodying the principles of the invention,

FIGS. 2A and 2B are sectioned views of a more complex transmissionembodying the principles of the invention,

FIG. 3 is a detail taken from FIG. 2 and drawn to a larger scale,

FIG. 4 is a view similar to FIG. 2 of a portion of the transmission, thesection being taken on a plane angularly offset from the plane of thesection in FIG. 2,

FIG. 5 is a plan view of a simple jig for use in assembly of thetransmission shown in FIGS. 2 to 4, and

FIGS. 6A and 6B are diagrams for use in explanation of the constructionof the transmission shown in FIGS. 2 to 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 10indicates an input shaft mounted in bearings 12 so as to be co-axialwith an output shaft 14 mounted in bearings 16. As will be describedbelow, torque can be transmitted from input shaft 10 to output shaft 14either by way of a direct coupling between those shafts, or by way of apair of intermediate shafts 18, mounted in respective bearing sets 22,24. All the shafts are axially and radially constrained.

To enable transmission of torque by way of the intermediate shafts, thetransmission comprises two gear assemblies. One gear assembly comprisesinvolute, helical gears 26, 28 on respective intermediate shafts 18, 20and conjugate gear 30 which is mounted on output shaft 14 by means ofbearings 32 permitting free rotation of the gear relative to the shaft.The second gear assembly comprises involute, helical gears 34, 36 onrespective intermediate shafts 18, 20 and conjugate gear 38 which isinternally meshed with input shaft 10 in a manner to be described infurther detail below.

Each gear is in constant mesh with the associated gear(s) in itsassembly. Thus, intermediate shafts 18, 20 are always rotated by thesecond gear assembly in use. However, torque can only be transmittedfrom shafts 18, 20 to. output shaft 14 by way of internally splinedclutch ring 40. This ring is mounted so as to permit axial slidingmotion on an externally splined member 42 which is rotatable with shaft14. Ring 40 can thus be axially moved into mesh with splines 44 on gear30. Direct coupling of shaft 10 to shaft 14 can be effected by axialsliding of ring 40 into mesh with externally splined portion 46 mountedfor rotation with shaft 10. Clutches employing sliding rings similar toring 40 are well known in the art, and accordingly it is thoughtunnecessary to provide further detail of this arrangement, or the meansfor effecting axial sliding of the clutch ring.

Consider now the situation in which torque is being transmitted fromshaft 10 to shaft 14 via the intermediate shafts 18 and 20. At eachregion of mesh within each gear assembly, the forces on the gears willact in a direction substantially normal to the areas of contact betweenthe gears. Since all the gears are helical, each of these forces has acomponent in a direction parallel to the central axis of thetransmission. Gear 38 is therefore subjected to such axial components oneither side of the central axis of the transmission. If these axialcomponents are not substantially equal that is, if the shafts 18 and 20are not carrying substantially equal loads, there will be a turningmoment applied to gear 38, tending to rotate it about an axissubstantially normal to the plane of FIG. 1 and passing through thecentral axis of the transmission. In order to permit substantialequalization of these forces, and thereby substantial equalization ofthe loads carried by shafts 18 and 20, gear 38 is mounted on shaft 10,in a manner described in detail below, such as to permit the gear topivot about the above-mentioned axis normal to the plane of the figure.

Consider the situation in which gear 30 is clutched to shaft 14, thelatter is clutched to a load and drive torque is applied to shaft 10.Assume that gear 38 first engages gear 34, there being a clearancebetween gear 38 and gear 36. Shaft 18 rotates to bring gear 26 intoengagement with gear 30, thereby tending to rotate shaft 14, and as aresult a corresponding axially directed reaction force is developed onthe right hand side of gear 38 as viewed in FIG. 1. Since there is nobalancing force developed on the left hand side of this gear, it pivotsuntil gear 38 is brought into contact with gear 36. Shaft 20 and shaft14 will now begin to rotate, and if gear 28 is engaged with gear 30,then the load will be shared between the two shafts, the axiallydirected reaction components on gear 38 will balance, and the gear willcease pivoting.

If gear 28 is not in engagement with gear 30 when gear 38 engages gear36, then shaft 20 will start to rotate before shaft 14. There will stillbe no axially directed components on the left hand side of gear 38because shaft 20 has not yet taken up anyload. Gear 38 will thereforecontinue to pivot anti-clockwise, its teeth sliding over the teeth ofgear 36 to cause the rotation of that shaft. When gear 28 comes intoengagement with gear 30, the pivoting and sliding cease as shaft 20takes up its share of the load and the axially directed components ongear 38 come into balance.

As rotation of the various shafts continues, errors in the formation ofthe individual gears or their associated shafts, or in the mounting ofthe various parts of the asembly, may tend to again throw the load ontoone or other of the intermediate shafts. Any such tendency will beaccompanied by imbalance in the axial reaction components on gear 38,causing pivoting of that gear in a direction appropriate to restore loadsharing betwen the intermediate shafts in the manner described above.

The mounting of gear 38 is designed to restrain the gear against axialbodily movement under the axial components referred to above, whilepermitting limited pivoting movement. Two features of the illustratedmounting are important in this connection, namely i. axial location ofthe gear by clamping it between annular blocks 48 of compressible,elastomeric material, and

ii. special formation of the external teeth on shaft as indicated at 50.

This formation can be achieved by barrelling the teeth, or bycrown-shaving them. Neither of these features is essential to asatisfactory gear mounting, but the use of elastomeric blocks isparticularly advantageous, in that, besides achieving the axial locationwith limited pivotal freedom requirement, it also serves to cushion thetransmission against shock loads and to reduce noise.

Gears 26, 28 and 32 could be spur gears. However, helical gears arepreferred because they have more satisfactory operationalcharacteristics than spur gears, particularly with regard to noise andsurface stress fa tigue capacity.

It should be noted that the pivoting movements made by gear 38 areminute they are required only to take up the minute errors which occurin manufacture and assembly of the gearbox. Nevertheless, it isdesirable to reduce, where possible, the angle through which gear 38 isrequired to pivot. Aside from accuracy of manufacture and assembly ofthe parts of the transmission, one way of reducing the angle of pivotrequired of gear 38 is to arrange the transmission so that eachintermediate shaft would carry the load for at least part of one fullrotation of gear 30, even if gear 38 were not capable of driving thegears of the first assembly into continuous simultaneous contact. Ifthis can be arranged, gear 38 will be required to pivot both clockwiseand anticlockwise, as viewed in FIG. 1, in the course of one rotation ofgear 30, but the extent of pivoting in either direction will be lessthan if pivoting were required in only one direction.

In order to achieve this, it is necessary to arrange the transmission sothat at least one point in each rotation of gear 30, all the gears arein simultaneous contact even without pivoting of gear 38. Preferably,the arrangement is such that simultaneous contact is achieved at somemean" point, so that gear 38 pivots through approximately equal anglesin each direction from its orientation at this mean point.

Consider now the transmission shown in FIG. 1 immediately after firstassembly. Assume a holding torque is applied to shaft 14, and a drivetorque is applied to shaft 10, the value of this drive torque beinginsufficient to cause pivoting of the gear 38 against the resistance ofthe elastomeric blocks. Assume further that contact is first establishedin the gears on shaft 18, there being clearance between the intercalatedteeth of gears 38, 36 and gears 28, 30. Then shaft will be free torotate between limits determined respectively by contact between gears38, 36 and contact between gears 28, 30. This angular play can bemeasured. It will vary with the angular orientation of each shaft in theassembly, and a number of measurements can be taken at variousorientations. The gears can be initially as- 1 sembled so that play isalways found in shaft 20 whatever the angular orientations of thevarious shafts.

Now, it an average play for a number of angular orientations isestablished, this angular play can, by reference to the lead of gear 28,be converted into an axial adjustment of that gear required to take upthe average play. This is a property of helical gears, and provides.

the advantage that even after the gears have been formed, they can beadjusted by simple axial movement into a required contact relationship.The axial adjustment can be achieved by shifting gear 28 along its shaftand re-locating it with a specially formed setting shim. Alternatively,the gear can be intially located by a special setting collar, which canbe removed and replaced by a specially formed locating collar afteradjustment of the gear.

The effect of the elimination of the average play referred to above isto achieve the required mean si multaneous contact, so that, thereafter,in the absence of pivoting gear 38, load would be transferred from oneintermediate shaft to the other at about the mean point.

FIGS. 2 and 3 show a more complex transmission housed within a casing60. The transmission comprises an input shaft 62 which is mounted in oneend wall 63 of casing 60 by means of ball bearings 64. The transmissionalso comprises an output shaft 66 mounted in the other end wall 67 ofcasing 60 by means of conical roller bearings 68. Within the casing 60 afurther shaft 70 is mounted co-axial with input shaft 62 and outputshaft 66 by means of bearings 72, 74 in internal walls 76, 78 of thecasing. Wall 78 divides the interior of the casing into a main sectionof the transmission to the left of the wall 78 as viewed in FIG. 2, anda range section to the right of the wall 78 as viewed in FIG. 2.

A pair of intermediate shafts 80, 82 are mounted in respective bearings84, 86 in the end wall 63 of the casing, and in respective bearings 88,90 in interior wall 76. As best seen in FIG. 4, these intermediateshafts extend through openings in interior wall 78 and are mounted attheir ends in respective bearings 92, 94 in a further interior wall 96within the range section of the transmission. Intermediate shafts 80, 82extend parallel to, and are equiangularly spaced around, shaft 70.

Within the range section of the transmission, a pair of intermediateshafts 98, 100 extend between the interior wall 78, and the end wall 67of the casing, being mounted in respective hearings in each of thosewalls. Shafts 98, 100 extend parallel to, and are equiangularly spacedaround the inwardly projecting end of shaft 66. The axes of shafts 98,100 are, however, angularly offset from the axes of shafts 80, 82.

Shafts 80, 82 carry respective single, involute, helical gears 102, 104which mesh with a single, involute, helical gear 106 mounted on inputshaft 62. The mounting for gear 106 comprises a part spherical bearing108, and a pair of elastomeric rings 110 engaging respective annularfaces on the gear. The rings and the gear are clamped between a shoulder112 on the input shaft 62 and a nut 114 co-operating with an externallythreaded portion on the end of the shaft. Shoulder 1 12 has externalhelical teeth 113 to co-operate with internal teeth 116 on the gear 106,so that the gear is driven by the input shaft 62. These teeth are crownshaved so that they, the bearing 108 and the elastomeric rings 110provide freedom for pivoting of the gear 106 to permit substantialequalisation of load between shafts 80, 82.

The lead of teeth 116 is made equal to the lead of the teeth 117 on gear106. Thus, the loading of the rings 110 will be minimized because theaxial components on teeth 116 will be substantially balanced by theaxial components produced on teeth 117.

Within the main section of the transmission, shafts 80, 82 each carryfour gears 118, 120, 122 and 124. These provide four forward speedsderivable from the transmission, a fifth forward speed being obtained bydirect coupling of input shaft 62 to shaft 70 as described below. Withinthe range section of the transmission, shafts 80, 82 each carry a gear126 which is used, in a manner to be described below, to give reverseoperation of the transmission. Within the main section of thetransmission, shaft 70 carries gears 128, 130, 132 and 134 meshedrespectively with gears 118, 120, 122 and 124.

Consider first forward operation of the gear box, ignoring for themoment the range gears to the right of the wall 78. Assume, therefore,that shaft 70 is coupled directly to output shaft 66 by'means of theclutch ring 150, which would be moved to the left from the positionshown in FIG. 2 to enable this connection. Now, the input shaft 62 canbe coupled directly with the shaft 70 by means of the clutch ring 152,which would be moved to the left relative to the position shown in FIG.2. This would give a connection straight through the transmission. Sincegear 106 is permanently meshed with the input shaft, and with gears 102,104, both of the intermediate shafts 80, 82 would be rotated, butneither would transmit load because each of gears 128, 130, 132 and 134is free to rotate on shaft 70.

If it is now desired to drive shaft 70 via the intermediate shafts,clutch ring 152 is moved clear of input shaft 62, and either that clutchring is moved into mesh with gear 128, or clutch ring 154 is moved intomesh with either gear 130 or gear 132, or clutch ring 156 is moved intomesh with gear 134. Meshing of one of these clutch rings with one of thegears on shaft 70 applies the load to at least one of the shafts 80, 82thus resulting in production of axial reaction forces on the gear 106,which forces, if unbalanced, will cause pivoting of that gear about anaxis normal to the plane of FIG. 2 and passing through the center of thespherical bearing. Thus, load will be substantially equally sharedbetween shafts 80, 82 as'described above with reference to FIG. 1.

The range section of the transmission enables further variation of theoutput speeds obtainable from the transmission. When the range sectionis in use, clutch ring 150 would be moved out of engagement with shaft70. That shaft is provided within the range section with a gear 136meshed with gears 138 on respective shafts 98, 100. The latter shaftsalso each carry a double helical gear comprising portions 140, 142carrying oppositely directed helical teeth. Portions 140, 142 are meshedrespectively with single helical gears 144, 146 on the inwardlyprojecting portion of output shaft 66. Gears 144, 146 are internallysplined to co-operate with respective externally splined portions 149,151 on a sleeve 153 which is journaled on the shaft 66. Gears 144, 146are clamped between elastomeric rings 155 similar to the elastomericrings 110. The rings are themselves clamped between outwardly extendingflanges on the sleeve 153, the flange adjacent clutch rings 150 beingsplined to co-operate with that clutch rmg.

To understand operation of the range section of the transmission, it issufficient to consider the shaft as the input shaft to that section.Drive to the shaft 70 may have been derived from any of the abovedescribed modes of operation of the main section of the transmission.Since gear 136 is permanently meshed with gears 138, shafts 98, will berotated even when clutch ring is engaged with shaft 70. However, thiswill merely cause sleeve 153 to rotate on shaft 66, since there willthen be no coupling between the sleeve and the output shaft. However, ifclutch ring 150 is moved to the right to couple with the splines onsleeve 153, then load will be applied to at least one of theintermediate shafts 98, 100 and then will be substantially equallyshared between them because of the mounting of the output gears 144, 146on the shaft 66.

Since gears 144, 146 are helical, each is subjected to axial reactionforces in the same way as input gear 106. Each is mounted so as to giveit limited freedom to pivot about a respective axis normal to the planeof FIG. 2. It is to be expected that, in use, contact will first occurbetween one portion of one double helical gear, and the correspondingoutput shaft gear, for example between the gear portion 140 in the lowerhalf of FIG. 3, and the gear 144. This will create a reaction forcetending to push gear 144 to the right relative to the shaft 66, and topivot it in an anti-clockwise direction about the above mentioned axis.These reaction forces will be transferred through the interveningelastomeric rings to the gear 146, and the rightward movement of gear144 will continue until gear 146 is forced into contact with one or bothof gear portions 142. When this happens, further reaction forces will beset up to create compression of the elastomeric rings 155 so that eachgear 144, 146 attains a zone of contact with each of its correspondinggear portions 140, 142. The double helical gear assists in reducingstress on the teeth of the output gears, while reducing axial forces onthe bearings holding shafts 98, 100.

The reverse mode of operation will now be described. As mentioned above,this involves use of the gears 126, each of which is provided on asleeve (FIG. 4) which projects through the associated opening in thewall 78. Sleeve 160 is mounted on its associated shaft 80 or 82 by wayof needle roller bearings. The end of each sleeve 160 within the mainsection of the trans mission is externally splined at 162 toco-operatewith a respective clutch ring 164 by which the sleeve can beclutched to the associated shaft 80 or 82. It will be noted from FIG. 4that the external diameters of gears 126, 136 are such that they are notmeshed. However, each gear 126 is meshed with its adjacent gear 138 onrespective shafts 98, 100.

When reverse gear is not in use, clutch rings 164 adopt the positionshown in FIG. 4, so that sleeves 160 rotate freely on their respectiveshafts 80, 82. When reverse gear is engaged, clutch ring 152 is moved tothe left so that input shaft 62 is clutched to gear 106, and load istransmitted via the shafts 80, 82 which rotate in the same direction asin forward drive. However, clutch rings 164 are moved to the right so asto drive sleeves 160 and gears 126. The latter gears drive gears 138 andthus shafts 98, 100, and gears 144, 146. Clutch ring 150 is moved to theright to mesh with sleeves 153, thereby to drive shaft 66. Reversal isachieved by the inclusion of additional gears 126 in the train. Due topermanent meshing of gears 1 36, 138, shaft 70 is rotated, but is freeto do so since it is not clutched to any of the gears 118, 120, 122 and124. It will be apparent that since both input gear 106 and output gears144, 146 are used in the reverse mode, load sharing on shafts 80, 82 andon shafts 98, 100 is achieved in this mode also.

As is the case of the simple transmission shown in FIG. 1, it isdesirable to reduce, where possible, the angle through which the pivotalgear is required to pivot in use. As before, this can be done byachieving mean simultaneous contact, but in this case it must beachieved independently for each gear assembly. Accordingly, each gear128, 130, 132, 134 is clutched to a holding torque in turn, and for eacha drive torque in applied and a number of free play measurementsobtained. Suitable locating collars are then used, as before, to ensurethat for each gear assembly the gears are in the appropriaterelationship.

In order to enable assembly of this transmission, it is necessary toensure that the intermediate shaft gears are appropriately locatedaxially on their respective shafts before the latter are mounted in thecasing. To achieve this, it is desirable to control closely the axialdimension of each gear on an intermediate shaft. A convenient means ofachieving such control will now be described with reference to FIG. 5.

FIG. shows a typical involute helical gear to be assembled on anintermediate shaft. It is mounted in a jig comprising a base plate 200,an upstanding wall 202 having an accurately formed surface 204. Fromsurface 204 there projects a stub shaft 206 formed with a key way 208similar to the intermediate shaft on which the gear is mounted in use. Asphere 210 is arranged such that one of its axes is accurately alignedwith the center line of key way 208 at a predetermined distance fromsurface 204. The sphere is vertically movable towards and away from thekey way. While the sphere is moved away from the key way, the gear ismounted on the stub shaft, and the sphere is then moved back towards thekey way to engage two teeth of the gear as shown in FIG. 5. Because ofthe geometry of the gear and the sphere, the abovementioned axis of thesphere is accurately aligned with a central axial plane of the gear,which is thereby accurately spaced from surface 204.

As seen in FIG. 5, the gear is formed with an axially projecting boss212. During the manufacturing procedure, this boss is deliberatelyformed over size in the axial dimension so as to allow the possibilityof finish machining of the free axial surface of the boss to desiredspacing from the abovementioned central axial plane of the gear. Therequired amount of machining to achieve the required spacing can bedetermined by measurement of the clearance between the axial face of theboss and the surface 204.

The transmission shown in FIGS. 2-4 includes a further feature to ensuresharing of torque between the intermediate shafts 80, 82 on overrun.Consider the assemblies shown diagrammatically in FIGS. 6A and 6B. FIG.6A shows the assembly including gear 134, the largest diameter gear onshaft 70. FIG. 6B shows the assembly including gear 128, the smallestdiameter gear on shaft 70. The arrow shows the direction of rotation ofthe gears in all modes of operation.

Assume that gear 134 is clutched to shaft 70. Then FIG. 6A shows thegears in the normal driving condition, with simultaneous contactachieved at y y and z -z that is, with both shafts 80, 82 driving gear134 and shaft 70. Gears 118 (FIG. 68) will also rotate gear 128 which isfree relative to shaft 70. These gears have been illustrated withsimultaneous contact at w w and x x They will not necessarily be in thiscondition, since they are not subject to the balacing effect of thepivotal gear, but the condition shown in FIG. 6B will be achievedperiodically during rotation of shafts 80, 82 because the initialadjustment: procedure has ensured it as described above.

Assume now that the transmission goes into overrun, that is, shaft 70becomes the driving shaft. Then teeth y and 2 will close on teeth y andz, respectively, but due to inevitable errors of the type discussedabove, contact will be made on one side of gear 134 first, say at zz1.Therefore, shaft 80 must rotate independently of shaft 82 through anangle given, in radians, by dmt/r, where dmt is the difference betweenthe transverse tooth clearances, i.e. clearances in the plane ofrotation, y-y and z-z and r (FIG. 6A) is the meshing radius of gears124.

During this independent rotation of intermediate shaft 80, the shaft 80carries with it the whole series of gears it carries, including gear 118shown on the left of FIG. 68. But, the meshing radius of gear 118 is r(which may be about Zfatimes r) and a pointon this meshing radius willmove through an arc of length r /r times the arc movement of a point onmeshing radius of gear 124. The required independent rotation of shaftis only possible if sufficient transverse clearance exists at xx sinceif contact is established there before contact is established at ZZ1,then a clearance will remain at z-z, and torque cannot be shared betweenthe two intermediate shafts on the overrun.

The maximum actual difference between transverse clearances at y-y andzz will depend on the tooth thickness tolerances of the gears, and oncenter distance tolerances. However, as explained more fully below, themaximum effective difference in transverse clearances will equal thetooth thickness tolerances on the intermediate shaft gears, sincedifferences in the respective center distances apply to all gears on anyintermediate shaft, assuming that the shafts are parallel.

Neglecting center distance variations, there are three conditions of thegears shown in FIG. 6A which ought to be considered, namely (1) allgears in maximum metal condition, (2) all gears in minimum metalcondition, (3) one gear 124 in maximum metal condition and the other inminimum metal condition. In cases I) and (2), the transverse toothclearances will be equal, though they will differ as between the cases.In neither case is there any need for rotation of one shaftindependently of the other, and no binding problem arises. The worstcast is case (3) but it must be borne in mind that the tooth toleranceson central gear 128 affect transverse tooth clearances at both gears118. Thus, the maximum difference between the clearances at oppositeregions of mesh on gear 128 is equal to the transverse tooth toleranceson gears 118.

Referring now to FIG. 6B, sufficient clearance must be provided at x-xto permit said independent rotation of shaft 80. If r is 2.5 times r,then clearance needed at x-x is at least 2% times the transverse toothtolerances on gears 124. In addition to this minimum clearance, someallowance must also be made for cyclic and alignment errors to ensurefreedom for unclutched gears. The design backlash may be insufficient topermit the required independent roation.

Since helical gears contact normal to helicoidal surfaces, the tooththickness tolerances are given and measured in the normal plane. In theplane of rotation, however, tooth clearances will depend on chosenvalues of normal pressure and helix angles. By choosing appropriatenormal pressure and helix inclinations which not only satisfy the primedesign requirements so far as surface stress and strength capacity isconcerned the additional transverse clearance necessary to preventbinding can be provided. In practice, the appropriate clearance is mostconveniently obtained by varying the helix inclinations of the gearteeth along the intermediate shafts.

If, for example, the gears shown in FIG. 6A have normal pressure angleof 16 and design helix angle of I6", whilst the gears shown in FIG. 68have normal pressure angle of 16 but helix angle of 45, and if normaltooth thickness tolerance and required minimum normal backlash are thesame for both sets of gears, then sufficient clearance can be providedto prevent binding on overrun as follows:

Normal tooth thickness tolerances (Assume) 000" 0025" Minimum normalbacklash (Assume) .006" Maximum difference of backlash in the plane ofrotation:

.0025 .0025 H cos 16 X cos 16 .923 0027 Required minimum clearance planeof rotation: .0027 X 2.5

- '9L Actual minimum clearance at x x, C 0 X COS The word normal heremeans normal to the tooth surfaces, and is not to be confused withdesign backlash, tolerances, etc.

Consideration of the above calculations will show that the greater helixangle is required closer to the input end of the transmission. Thus, thehelix angle of the teeth on gears 118 may approach 45.

The invention is not limited to details of the embodiments describedwith reference to the drawings. For example, the part spherical bearing108 (FIGS. 2 and 3) may be omitted. Centering of the gear 106 relativeto the input shart 62 can be achieved automatically by its driveconnection with teeth 116. Further when the part spherical bearing isprovided, the rings 110 could be omitted, but it is preferable toprovide these because of the additional advantages which accompany theiruse, as described with reference to the similar elastorneric blocks inFIG. 1.

It is desirable to form teeth 116 as helical teeth to control thecompression forces applied to the rings 110 as described above. However,this is not essential since 12 the gear 106 could be driven by spliningit to input shaft 62. Further, the end of shaft 62 could be providedwith a bevel gear, co-operating with a similar bevel portion on the gear106. The teeth of the two bevel gears could be specially formed in acam-like fashion, to permit the pivoting movement of the gear 106relative to the shaft 62. The bevel gears may be provided in a Curvic(Registered Trade Mark) coupling. Again, gear 106 would preferably beaxially restrained by an elastomeric ring to achieve the advantagesdescribed above. In any embodiment, each ring is preferably backed by arigid steel backing member.

The embodiments have been described using involute helicoidal gears.These gears are preferred because of operational characteristics whichare well known and understood in the gear design art. However, the sameprinciples are applicable if screw or convolute helicoids are used.

It is not necessary for a transmission in accordance with the inventionto achieve equal load sharing between the intermediate shafts in orderto obtain sub stantial advantages. The actual share taken by eachintermediate shaft depends upon the effective center of pivot of thepivotal gear. If this is offset from a position centrally locatedbetween the intermediate shafts, then the shafts will carry unequalloads. On the other hand, an arrangement in which the center of pivot isoffset from a central location between the intermediate shafts canreduce the degree of pivot required if the pivotal gear is required topivot in only one direction to achieve load sharing.

Any arrangementwhich ensures that a substantial portion of the totalload is carried by each intermediate shaft will in itself provideadvantages for the transmission designer. For example, an arrangement inwhich two-thirds of the total load is carried by one intermediate shaft,and one-third by the other, may be satisfactory. Capacity of gear teethis characterised by an endurance limit level of stress. If calculatedstresses fall below endurance levels, then the design can be consideredsatisfactory. There is an exponential relationship between stress andexpected life, and accordingly any reduction of stress can have adramatic effect on life expectancy.

I claim:

1. A transmission comprising first and second rotatable members betweenwhich torque is to be transmitted, a first helical gear rotatable withsaid first member, a plurality of further helical gears meshed with saidfirst gear, coupling means for coupling said further gears with saidsecond member, and mounting means for said first gear arranged to permitsaid first gear to pivot about an axis transverse to the rotational axisof said first gear under the action of axially directed components ofreaction forces arising from said further gears under load so thattorque can be transmitted between the members through each of saidfurther helical gears.

2. A transmission as claimed in claim 1 wherein the mounting meanscomprises part spherical bearing surfaces.

3. A transmission as claimed in claim 1 wherein each of said furthergears has two portions of opposite helical inclination, the firsthelical gear also having two corresponding portions meshed withrespective portions of each further gear, said mounting means permittingeach portion of said first helical gear to pivot so that torque can betransmitted between said first and second members via each portion ofeach of said further helical gears.

4. A transmission as claimed in claim 1 wherein said first member is aninput member.

5. A transmission as claimed in claim 1 wherein said mounting meanscomprises axial locating means having a degree of resilience such as topermit said pivoting.

6. A transmission as claimed in claim 5 wherein said axial locatingmeans comprises elastomeric rings.

7. A transmission in accordance with claim 1 wherein said coupling meanscomprises a plurality of gear assemblies providing different ratios,each assembly comprising a plurality of gears fixed for rotation withrespective ones of said further gears and an associated gear meshed witheach of the plurality of gears in its assembly, and clutch means forselectively clutching said associated gears to said second member.

8. A transmission as claimed in claim 7 wherein each of said gearassemblies comprises helical gears.

9. A transmission as claimed in claim 8 wherein the helix angles of theteeth of said gear assemblies are different to facilitate torquetransmission via each of said further helical gears both from said firstmember to said second member, and from said second member to said firstmember, the larger the meshing radius of the gears of the plurality, thelarger the helix angle of the teeth of those gears.

10. A transmission as claimed in claim 8 wherein each gear assembly isindividually adjusted axially such that each of its plurality of gearscontacts the associated gear when said first helical gear is in apredetermined disposition.

11. A transmission comprising first and second members between whichtorque is to be transmitted; a plurality of intermediate shafts coupledwith one of said members; a plurality of gear assemblies each comprisinga plurality of helical gears fixed respectively on said intermediateshafts and an associated helical gear meshed with each of theintermediate shaft gears in its assembly, said gears of each of saidassemblies having different meshing radii from said gears of each of theother of said assemblies to provide different transmission ratios;clutch means for selectively clutching said associated gears to theother of said members; and torque sharing means for causing torque to betransmitted simultaneously through each of the intermediate shaft gearsof any assembly whose associated gear is clutched to said other member,the helix angles of the gears of each of said assemblies being larger asare the meshing radii of said intermediate shaft gears thereof so that,the larger said meshing radius, the greater the rotational free playbetween the associated gear and the intermediate shaft gears, saidrotational free play being such that an assembly whose associated gearis not clutched to said other member does not prevent transmission oftorque by each intermediate shaft gear of an assembly whose associatedgear is clutched to the other member.

12. A transmission for obtaining reversible drive from a source ofunidirectional, rotational drive comprising an input member forconnection with said source, an output member to be reversibly driven,an output member driving gear for driving said output member, first andsecond couplings between said gear and said input member arranged todrive said gear in opposite directions and clutch means for selectingwhich of said couplings is effective to drive the gear.

13. A transmission as claimed in claim 12, wherein said first couplingcomprises a first gear rotatable by said input member, a second gearmeshed with the first and a third gear meshed with said driving gear,the second coupling comprising a fourth gear meshed with the drivinggear, said clutch means being arranged to select which of said third andfourth gears is rotatable by said input member to drive said drivinggear.

14. A transmission in accordance with claim 13 and comprising aplurality of gear assemblies and selector means for selecting which ofsaid plurality is to provide said first and second gears, saidassemblies providing different gear ratios.

15. A transmission as claimed in claim 12, wherein said driving gear isone of a plurality of driving gears each arranged for driving saidoutput member, there being a corresponding plurality of first couplingsindividual to said driving gears and each comprising a respective gearrotatable by said input member, a first common gear meshed with each ofsaid respective gears and a second common gear meshed with each of thedriving gears, there being also a corresponding plurality of secondcouplings individual to said driving gears and each comprising a.respective further gear meshed with the associated driving gear, saidclutch means being arranged to select corresponding couplings for eachdriving gear and to determine. whether the driving gears are driven bythe second common gear or the respective further gears, the transmissionfurther comprising means for causing torque to be transmitted via eachof said driving gears simultaneously to the output member.

16. A transmission in accordance with claim 15 and comprising aplurality of gear assemblies and selector means for selecting whichassembly of said plurality is to provide each of said respective gearsand said first common gear, said assemblies providing different gearratios.

17. A transmission comprising a main section which includes an inputshaft, a main shaft, an intermediate shaft, gears on each of saidshafts, said gears on said intermediate shaft being constantly meshedwith respective gears on said input shaft and said main shaft, andclutch means for selectively clutching said input shaft gears to saidmain shaft to provide selected different drive ratios from said inputshaft to said main shaft; a range section which includes an input shaftdriven by the main shaft of said main section, an output shaft, anintermediate shaft, gears on each of said shafts of said range section,said range section intermediate shaft gears being respectivelyconstantly meshed with said range section input and output shaft gears,and clutch means for selectively coupling said range section input shaftto said range section output shaft either directly or via said rangesection gears; and reverse means comprising reverse gear means inconstant mesh with a gear of said range section and reverse clutch meansfor clutching said reverse gear means to said main section counter shaftto provide reverse drive to said output shaft.

18. A transmission according to claim 17, wherein said reverse gearmeans comprises a single gear in constant mesh with said range sectionintermediate shaft gear.

19. A transmission according to claim 18, including a plurality of saidintermediate shafts in each of said sections, and a correspondingplurality of said single reverse gears and of said reverse clutch meansfor clutching said single gears to respective ones of said intermediateshafts.

1. A transmission comprising first and second rotatable members betweenwhich torque is to be transmitted, a first helical gear rotatable withsaid first member, a plurality of further helical gears meshed with saidfirst gear, coupling means for coupling said further gears with saidsecond member, and mounting means for said first gear arranged to permitsaid first gear to pivot about an axis transverse to the rotational axisof said first gear under the action of axially directed components ofreaction forces arising from said further gears under load so thattorque can be transmitted between the members through each of saidfurther helical gears.
 2. A transmission as claimed in claim 1 whereinthe mounting means comprises part spherical bearing surfaces.
 3. Atransmission as claimed in claim 1 wherein each of said further geArshas two portions of opposite helical inclination, the first helical gearalso having two corresponding portions meshed with respective portionsof each further gear, said mounting means permitting each portion ofsaid first helical gear to pivot so that torque can be transmittedbetween said first and second members via each portion of each of saidfurther helical gears.
 4. A transmission as claimed in claim 1 whereinsaid first member is an input member.
 5. A transmission as claimed inclaim 1 wherein said mounting means comprises axial locating meanshaving a degree of resilience such as to permit said pivoting.
 6. Atransmission as claimed in claim 5 wherein said axial locating meanscomprises elastomeric rings.
 7. A transmission in accordance with claim1 wherein said coupling means comprises a plurality of gear assembliesproviding different ratios, each assembly comprising a plurality ofgears fixed for rotation with respective ones of said further gears andan associated gear meshed with each of the plurality of gears in itsassembly, and clutch means for selectively clutching said associatedgears to said second member.
 8. A transmission as claimed in claim 7wherein each of said gear assemblies comprises helical gears.
 9. Atransmission as claimed in claim 8 wherein the helix angles of the teethof said gear assemblies are different to facilitate torque transmissionvia each of said further helical gears both from said first member tosaid second member, and from said second member to said first member,the larger the meshing radius of the gears of the plurality, the largerthe helix angle of the teeth of those gears.
 10. A transmission asclaimed in claim 8 wherein each gear assembly is individually adjustedaxially such that each of its plurality of gears contacts the associatedgear when said first helical gear is in a predetermined disposition. 11.A transmission comprising first and second members between which torqueis to be transmitted; a plurality of intermediate shafts coupled withone of said members; a plurality of gear assemblies each comprising aplurality of helical gears fixed respectively on said intermediateshafts and an associated helical gear meshed with each of theintermediate shaft gears in its assembly, said gears of each of saidassemblies having different meshing radii from said gears of each of theother of said assemblies to provide different transmission ratios;clutch means for selectively clutching said associated gears to theother of said members; and torque sharing means for causing torque to betransmitted simultaneously through each of the intermediate shaft gearsof any assembly whose associated gear is clutched to said other member,the helix angles of the gears of each of said assemblies being larger asare the meshing radii of said intermediate shaft gears thereof so that,the larger said meshing radius, the greater the rotational free playbetween the associated gear and the intermediate shaft gears, saidrotational free play being such that an assembly whose associated gearis not clutched to said other member does not prevent transmission oftorque by each intermediate shaft gear of an assembly whose associatedgear is clutched to the other member.
 12. A transmission for obtainingreversible drive from a source of unidirectional, rotational drivecomprising an input member for connection with said source, an outputmember to be reversibly driven, an output member driving gear fordriving said output member, first and second couplings between said gearand said input member arranged to drive said gear in opposite directionsand clutch means for selecting which of said couplings is effective todrive the gear.
 13. A transmission as claimed in claim 12, wherein saidfirst coupling comprises a first gear rotatable by said input member, asecond gear meshed with the first and a third gear meshed with saiddriving gear, the second coupling comprising a fourth gear meshed withthe dRiving gear, said clutch means being arranged to select which ofsaid third and fourth gears is rotatable by said input member to drivesaid driving gear.
 14. A transmission in accordance with claim 13 andcomprising a plurality of gear assemblies and selector means forselecting which of said plurality is to provide said first and secondgears, said assemblies providing different gear ratios.
 15. Atransmission as claimed in claim 12, wherein said driving gear is one ofa plurality of driving gears each arranged for driving said outputmember, there being a corresponding plurality of first couplingsindividual to said driving gears and each comprising a respective gearrotatable by said input member, a first common gear meshed with each ofsaid respective gears and a second common gear meshed with each of thedriving gears, there being also a corresponding plurality of secondcouplings individual to said driving gears and each comprising arespective further gear meshed with the associated driving gear, saidclutch means being arranged to select corresponding couplings for eachdriving gear and to determine whether the driving gears are driven bythe second common gear or the respective further gears, the transmissionfurther comprising means for causing torque to be transmitted via eachof said driving gears simultaneously to the output member.
 16. Atransmission in accordance with claim 15 and comprising a plurality ofgear assemblies and selector means for selecting which assembly of saidplurality is to provide each of said respective gears and said firstcommon gear, said assemblies providing different gear ratios.
 17. Atransmission comprising a main section which includes an input shaft, amain shaft, an intermediate shaft, gears on each of said shafts, saidgears on said intermediate shaft being constantly meshed with respectivegears on said input shaft and said main shaft, and clutch means forselectively clutching said input shaft gears to said main shaft toprovide selected different drive ratios from said input shaft to saidmain shaft; a range section which includes an input shaft driven by themain shaft of said main section, an output shaft, an intermediate shaft,gears on each of said shafts of said range section, said range sectionintermediate shaft gears being respectively constantly meshed with saidrange section input and output shaft gears, and clutch means forselectively coupling said range section input shaft to said rangesection output shaft either directly or via said range section gears;and reverse means comprising reverse gear means in constant mesh with agear of said range section and reverse clutch means for clutching saidreverse gear means to said main section counter shaft to provide reversedrive to said output shaft.
 18. A transmission according to claim 17,wherein said reverse gear means comprises a single gear in constant meshwith said range section intermediate shaft gear.
 19. A transmissionaccording to claim 18, including a plurality of said intermediate shaftsin each of said sections, and a corresponding plurality of said singlereverse gears and of said reverse clutch means for clutching said singlegears to respective ones of said intermediate shafts.